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Blueprint of Life
Topic 7: Modern Genetics
Biology in Focus, HSC Course
DOT POINT(s)
 distinguish between the terms allele and gene, using
examples
 distinguish between homozygous and heterozygous
genotypes in monohybrid crosses
 explain the relationship between dominant and recessive
alleles and phenotype using examples
Introduction
On the basis of his discoveries,
Mendel proposed that each
kind of hereditary
characteristic is determined by
two ‘factors’ in every
individual. Mendel’s factors
are equivalent to what we call
genes today, but, to make this
advance, chromosomes and
their behaviour in mitosis and
meiosis had to be discovered.
www.biography.com
Introduction
This, combined with the
discovery of DNA (the chemical
of which chromosomes are made)
led to the field of study known
as genetics—the study of
heredity and variation.
jeanapettus.webs.com
Review
Assumed knowledge:
A gene:
 is a segment of DNA on a
chromosome
 specifies a particular
characteristic (e.g. seed
colour)
 has two alleles in an
individual and two or more
alternative alleles in a
population
en.wikipedia.org
Review
Assumed knowledge:
alleles:
 An allele is an alternative form of a gene
(one member of a pair) that is located at a
specific position on a specific
chromosome.
 segregate during gamete formation
(meiosis)
 occur individually in each haploid gamete
 pair during fertilisation, when the diploid
condition of an organism is restored
during zygote formation.
www.lhasa-apso.org
Alleles and Genes
In our studies of modern-day genetics, we know that cells
contain units of heredity known as genes on chromosomes.
Different genes influence different characteristics. For example
in pea plants, one gene may determine seed colour, while
another determines stem length. In humans, genes determine
characteristics such as height, eye colour, hair colour and
freckles.
www.educ.ttu.edu
Alleles and Genes
Each cell contains two copies of every gene, one inherited from
each parent. Different variations (contrasting versions) of the same
gene are termed alleles of that gene.
www.biography.com
Alleles and Genes
For example, the gene for height has two alleles—tall (T ) and
short (t). These versions of the same gene are found in identical
positions (loci) on a pair of similar chromosomes (homologous
chromosomes) within cells. Diploid individuals have two alleles
for each gene and haploid cells (gametes) have only one allele of
each gene.
www.biography.com
Alleles and Genes
Sometimes within a population there are more than two alleles
for a particular gene, e.g.:
 flower colour in sweet peas—pink, white, purple, red and so
on
 hair colour in Labrador dogs—black, brown or yellow
(golden)
www.ck12.org
Student Activity
www.biography.com
Genotypes: Homozygous and
Heterozygous
Mendel’s terms pure breeding
and hybrid are known in
modern genetics as
homozygous and
heterozygous respectively.
These terms are used to
describe the combination of
alleles present in a cell.
www.exploringnature.org
Genotypes: Homozygous and
Heterozygous
If both copies of the gene in a cell are the same the purebreeding individuals for this trait are, in modern terminology,
said to be homozygous, meaning that the organism has
identical alleles for a particular genetic trait (e.g. TT or tt). (The
term ‘homozygous’ is derived from two words: zygote = a
fertilised egg, having half its material from each parent; and
homo = the same).
www.plant.uoguelph.ca
Genotypes: Homozygous and
Heterozygous
If the alleles of a gene are contrasting or differ for that trait (e.g.
Tt), the hybrid individuals (as described by Mendel) are now
said to be heterozygous (hetero = different).
www.fao.org
Dominant and Recessive Alleles
The genetic make-up or
genotype of organisms,
described by the terms
homozygous and heterozygous,
determines the physical
appearance or phenotype (that
is, the genotype is the ‘cause’ of
the way it looks and the
phenotype is the ‘effect’—its
actual physical appearance).
fineartamerica.com
Dominant and Recessive Alleles
When a pair of differing alleles (e.g. Gg) occurs in an individual
(a heterozygote) and only one of the alleles is expressed (visibly
appears in the organism), this allele is known as the dominant
allele.
schoolworkhelper.net
Dominant and Recessive Alleles
The allele which is not
expressed (i.e. hidden or
masked) is called the
recessive allele. For example,
the allele for the short
characteristic (t) in pea plants.
These alleles are different
variations of the same gene.
The phenotype (appearance) of
an organism is determined by
the dominant (expressed)
genes.
sctritonscience.com
Dominant and Recessive Alleles
Recessive alleles like red hair in
humans, are often carried for several
generations without being expressed
(without appearing in the phenotype),
but may appear in a later generation.
This later appearance of recessive
traits is sometimes referred to as
‘skipping’ a generation (when masked
in the hybrid form) and only reappears
if it occurs in the homozygous
recessive form (as a result of a
monohybrid cross).
momjovi.com
Dominant and Recessive Alleles
In current genetic studies, phenotype is recognised as not only
the physical appearance of an organism, but may also include its
physiology (functioning) and aspects of its behaviour.
seniorapbiologyreview.wikispaces.com
Dominant and Recessive Alleles
Phenotype is predominantly determined
by genotype, but may also be influenced
or modified due to interaction with the
environment. For example the final
height of a human adult depends on a
combination of that person’s genotype as
well as their nutrition—if they are
underfed or lacking protein while
growing, they may never attain their full
potential height as determined by their
genotype. (This will be dealt with in
more detail in Chapter 3.)
www.infobarrel.com
Variation
Genes are considered to be the
units of selection in the neoDarwinian theory of evolution
by natural selection. The
concept of alleles being
different forms of the same
gene can account for the source
of variation that was lacking in
Darwin’s proposal of the
theory.
whenintime.com
Variation
The term variation applies to the differences in the
characteristics (appearance or genetic make-up) of individuals
within a population. Organisms in a population may vary in
appearance, physiology (functioning) and behaviour—that is,
they vary in phenotype.
hoffman.cos.ucf.edu
Variation
At the time when Darwin and
Wallace proposed their theory
of evolution by natural
selection, there was no
knowledge of what was
responsible for the differences
in individuals within a
population or of how such
characteristics could be passed
on from one generation to the
next.
www.gnxp.com
Variation
Today, with our knowledge from studies of genetics, we know
that the genotype of an individual (which alleles are present for
particular genes on chromosomes) determine characteristics
inherited.
www.tumblr.com
Variation
Furthermore, characteristics are carried from parents to
offspring on chromosomes by gametes (sex cells). When
gametes combine, they may bring together a different
combination of alleles for the same gene. For example, the gene
for eye colour from each parent.
writingtodistraction.blogspot.com
Variation
Since some genes have more than two varieties (alleles) within a
population (e.g. eye colour and hair colour in humans), these
genes further increase the variation that can be inherited in
individuals, an essential ingredient in the process of evolution.
www.sciencebrainwaves.com
Variation
Mendelian principles of inheritance, such as dominance and
segregation, apply to all eucaryotes, including humans. Although
there are some instances where there are deviations from exact
Mendelian inheritance (such as sex-linkage and co-dominance, which
will be dealt with later), in general Mendel’s laws apply when:
■ characteristics are inherited as individual units
■ characteristics are controlled by a single pair of genes showing
dominance.
Homework
-Students to complete Genetics Terminology Worksheet